Rate of change detection

ABSTRACT

A method of monitoring the rate of change of a first signal which includes the steps of periodically sampling the first signal, generating a first reference signal which for each period differs from the sampled amplitude of the first signal by a predetermined amount, comparing the amplitudes of the first signal and of the first reference signal, and generating an output signal when the amplitude of the first signal crosses the amplitude of the first reference signal. 
     The invention also provides apparatus for monitoring the rate of change of a first signal which comprises means periodically to sample the first signal, means to generate a first reference signal which for each period differs from the sampled amplitude of the first signal by a predetermined amount, and means to compare the amplitudes of the first signal and of the first reference signal and to generate an output signal when the amplitude of the first signal crosses the amplitude of the first reference signal.

This invention relates to the detection of the rate of change of a signal.

Fire detectors, for example, monitor the amplitudes of signals which are dependent on the concentration of combustion products in the atmosphere, or on the concentration of gases such as carbon dioxide which are given off during combustion. A fire detector is designed to generate an alarm signal when the monitored signal crosses a preset threshold value. Normally the monitored signal is subject to the effect of temperature and ageing, for example, and its amplitude can therefore drift. In addition, the amplitude of the monitored signal may be affected by causes unrelated to combustion. For example a fire detector which monitors the concentration of atmospheric carbon dioxide will detect the carbon dioxide exhaled by a number of humans and if the threshold level does not allow for variations produced by such causes the detector will spuriously generate an alarm signal.

It is therefore necessary to set the threshold level sufficiently far from the normal amplitude of the monitored signal to reduce the incidence of false alarms. Unfortunately this means that the sensitivity of the detector is affected and the possibility exists that a smouldering fire which may only gradually increase the carbon dioxide level is detected only when it bursts into flame and pushes the carbon dioxide level beyond the threshold. The effectiveness of the detector is therefore limited for it is most desirable to detect and start fighting a fire in its incipient stages.

It is an object of the present invention to provide apparatus for monitoring the rate of change of a signal. The apparatus finds particular application in overcoming the problems outlined in respect of fire detectors but its usefulness is not limited to fire detectors. The applicability of the invention is therefore not confined to fire detectors and the scope of the invention is to be interpreted in this sense.

The invention provides apparatus for monitoring the rate of change of a first signal which comprises means periodically to sample the first signal, means to generate a first reference signal which for each period differs from the sampled amplitude of the first signal by a predetermined amount, and means to compare the amplitudes of the first signal and of the first reference signal and to generate an output signal when the amplitude of the first signal crosses the amplitude of the first reference signal.

It is necessary to choose the period, and the predetermined amount, so that changes in the first signal amplitude due to known fluctuations and drift are ignored. This may necessitate the use of a relatively long period.

In such a case the sampling means may comprise timing means which generates a timing signal each period, oscillator means which produces pulses at a rate which is proportional to the amplitude of the first signal, counter means which is enabled for a fixed interval by each timing signal and which counts the pulses produced by the oscillator means, and means to convert the count in the counter means into an analogue signal.

Alternatively the apparatus may include timing means for enabling the sampling means and the sampling means may comprise clock means which produces pulses at fixed rate, counter means which counts the pulses produced by the clock means, means to convert the count in the counter means into an analogue signal, and means to compare the analogue signal to the first signal and to generate a signal which suspends operation of the counter means when the analogue signal and the first signal are in a predetermined relationship to one another, thereby causing the analogue signal to be held constant for the period.

On the other hand where a relatively short period is tolerable the sampling means may comprise means to integrate the first signal amplitude over each period.

The invention further provides a method of monitoring the rate of change of a first signal which includes the steps of periodically sampling the first signal, generating a first reference signal which for each period differs from the sampled amplitude of the first signal by a predetermined amount, comparing the amplitudes of the first signal and of the first reference signal, and generating an output signal when the amplitude of the first signal crosses the amplitude of the first reference signal.

The method may further include the step of comparing the first signal to a second reference signal of fixed amplitude and generating the output signal when the amplitude of the first signal crosses the amplitude of the second reference signal.

The invention is further described by way of examples with reference to the accompanying drawings in which:

FIG. 1 illustrates the principle of the invention, and

FIGS. 2 to 4 are block diagrams of different forms of the apparatus according to the invention.

In FIG. 1 the amplitude of a signal 10 is plotted as a function of time. As shown, the amplitude of the signal 10 decreases with time, but the rate at which the signal decreases is increasing. According to the invention the amplitude of the signal 10 is periodically sampled at sampling instants 12. A reference signal 14 is generated, in step-wise fashion, which has an amplitude for each period T, between sampling instants, which differs from the sampled amplitude of the signal 10, at the beginning of the period, by a gap voltage 16.

Thus the amplitude of the reference signal tracks the amplitude of the signal 10 in step-wise fashion and is updated with respect to this signal after each period T. Consequently if during a period T the rate of change of the signal 10 exceeds the average maximum slope of the reference signal, i.e. the gap voltage 16 divided by the period T, the signal 10 will cross the reference signal 14. Thus if the signals 10 and 14 are constantly compared an alarm signal can be generated when such a crossing occurs.

FIG. 2 is a circuit diagram of apparatus which is used to generate the reference signal and compare it to the signal 10 when the period T is small. In other words, the circuit of FIG. 2 can be used to monitor the signal 10 when the fluctuations in the signal due to drift or other known causes are relatively rapid.

The circuit of FIG. 2 consists of an operational amplifier 18 to which are connected a resistor R and a capacitor C so that it functions as an integrator, a difference amplifier 20, and a comparator 24. The inverting input of the amplifier 20 is connected to a variable voltage source 26.

The time constant, RC, of the integrator 18 is chosen to equal the sampling period of the signal 10. The voltage tapped from the voltage source 26 equals the gap voltage 16. Thus the output voltage of the subtractor 20 equals the signal 10 integrated over a period T minus the gap voltage. This voltage is compared in the comparator 24 to the signal 10. When these signals cross one another, the comparator generates an alarm signal at its output.

FIG. 3 illustrates apparatus of the invention which may be used when the period T is relatively long. The apparatus consists of a voltage controlled oscillator 28, an AND gate 30, a clock 32, a counter 34, a digital to analogue converter 36, a variable voltage source 38, a subtractor 40 and a comparator 42.

At the beginning of every period T the clock 32 enables the gate 30 for a fixed relatively short interval. The output signal of the oscillator 28 is at a frequency which is directly proportional to the amplitude of the signal 10 and is applied to the counter 34 for this interval. The counter produces a count which is dependent on the frequency of the signal from the oscillator 28 and hence is directly proportional to the amplitude of the signal 10. This count is converted into an analogue signal by the digital to analogue converter 36 and the gap voltage 16 tapped from the voltage source 38 is subtracted from the analogue signal in the subtractor 40 to produce the tracking reference signal 14. Since the count in the counter 34 is updated every period T, and in a relatively short time, the signal 14 has a fixed amplitude for each period T. The oscillator 28, the counter 34 and the digital to analogue converter 36 thus function as a sample and hold circuit. The signals 10 and 14 are compared in the comparator 42 and an alarm signal is generated when the signals cross one another.

FIG. 4 illustrates a modification of the apparatus of FIG. 3, and like numerals are used in the two drawings to depict like components. The voltage controlled oscillator 28 and AND gate 30 of the apparatus of FIG. 3 are replaced in the apparatus of FIG. 4 by a comparator 44, a flip-flop 46 and a clock 48.

The comparator 44 is used to compare the signal 10 and the signal output by the digital to analogue converter 36.

At the beginning of each period T the clock 32 sets the flip-flop 46. The clock 48 is thereby enabled. It is free running and the pulses produced by the clock are accumulated in the counter 34. The count is converted into an analogue signal by the digital to analogue converter 36 and the output signal of the converter 36 is compared to the signal 10 in the comparator 44. When the two signals are equal the output signal of the comparator resets the flip-flop 46 and the clock 48 is stopped. In other respects the operation of the apparatus is similar to the operation of the apparatus of FIG. 3.

As previously mentioned, the invention finds particular application to fire detectors but its use is not to be restricted in this respect. Although the period T and the gap voltage 16 are chosen to take account of known fluctuations in the signal 10, it may happen, for example in a fire detector, that the rate of change of the signal 10 will be so low that the reference signal 14 will never be crossed. In such a case it is necessary to have a fixed threshold signal, for example as indicated by the reference numeral 50 in FIG. 1, which is compared continuously to the amplitude of the signal 10 so that an alarm signal is generated when the signals 10 and 50 cross.

The way in which this may be done is shown in FIG. 4. This drawing shows a fire detector 52 and a comparator 54.

The fire detector may be any suitable device and may for example be of the ionization type, with one or two chambers, or of the gas analyzing type. Such a detector produces an output signal 10 the amplitude of which is dependent on the concentration of one or more combustion products in the atmosphere. The signal 10 is applied to the apparatus of FIG. 4 and is monitored in the manner described. It is also compared in the comparator 54 to the fixed amplitude signal 50 and the alarm signal is generated when the signals 10 and 50 cross. 

We claim:
 1. Apparatus for monitoring the rate of change of a first signal which comprises:means for periodically sampling the amplitude of the first signal to produce a sampled signal, means responsive to the sampled signal for generating a reference signal which for each sample period differs from the sampled amplitude of the sampled signal by a predetermined amount, means for comparing the amplitudes of the first signal and the reference signal and to generate an output signal when the amplitude of the first signal crosses the amplitude of the reference signal, said sampling means comprising clock means which produces pulses at a fixed rate, counter means which counts the pulses produced by the clock means, means to convert the count in the counter means into an analogue signal, said analogue signal being said sampled signal, and means to compare the sampled signal to the first signal and to generate a signal which suspends operation of the counter means when the sampled signal and the first signal are in a predetermined relationship to one another, thereby causing the sampled signal to be held constant for the sample period.
 2. Apparatus according to claim 1 which includes means to compare the first signal to a second reference signal of fixed amplitude and to generate the output signal when the amplitude of the first signal crosses the amplitude of the second reference signal.
 3. Apparatus according to claim 2 which includes a fire detector, the fire detector producing the first signal which is dependent on the presence in the atmosphere of combustion products. 